An Experimental Study of Interfacial Dynamics Control Using Temperature-Sensitive Surfactants

The dynamic control of meniscus curvature is a fundamental aspect of interfacial science with broad implications for microfluidics, thermal management, and adaptive surface technologies. Here, we investigate the temperature-induced interfacial dynamics of menisci in oil-water systems containing thermoresponsive surfactants. Using high-resolution bright-field microscopy, we systematically examine the effects of temperature-sensitive surfactants, concentration, capillary geometry, temporal temperature gradients, contact angle hysteresis, and heat transfer direction (heating/cooling) on meniscus displacement, contact angle, and interfacial tension. Our results demonstrate that the combination of C₁₈TAB in water and hexadecanol in hexadecane enables reversible and tunable switching between concave and convex meniscus shapes, with the transition temperature (T_t) strongly dependent on surfactant composition and concentration. A unique thermal hysteresis in contact angle is observed and is attributed to the asymmetric adsorption/desorption kinetics and interfacial freezing, which is amplified in wider capillaries. However, temporal temperature gradients within the tested range have minimal influence on curvature switching, indicating quasi-static interfacial equilibration at supra-CMC concentrations. These findings establish a framework for actively manipulating interfacial geometry, providing design principles for programmable and reversible wettability control in interfacial systems.